A lethal batch of synthetic cannabis is suspected of contributing to the death of a youth in the Hunter Valley region and causing two others to become critically ill in a neighbouring suburb.

What was initially suspected of being the accidental death of popular teen Dean Shield, who was found unresponsive in a drain on Saturday night, has turned into a major investigation as detectives work on whether it is connected with the 18-year-old experimenting with a form of synthetic cannabis.

Dean's body was found by a resident in Rutherford, near Maitland, several hours after he is believed to have bought and taken some of the controversial substance.